A sackung structure can be a trench (small-scale graben) or an upslope-facing scarp. They are found most commonly in Alpine glaciated regions near the crest of a range. Their origin is interpreted to be a result of post-glacial, gravitational spreading of the ridge crest and over-steepened ridge flanks. The plural of this German word is sackungen.

Keystone Mountain Sackung sackung-Trail-Ridge

Left: Uphill-facing sackung scarp (yellow arrows) on Keystone Mountain. Photo by J. Stanesco

Right: A sackung is a linear scarp that usually parallels an elongate ridge and faces uphill. In general, the valleys on either side of the ridge have been carved and over-steepened by glaciers. Trail Ridge Road Sackung, Photo by V. Matthews

In searching for evidence of young faulting in Colorado, it is possible to be fooled by sackungen scarps that may on first examination appear to be young tectonic features. Although it is possible that these features might be used to understand major earthquakes on nearby tectonic faults, it is not believed that these features, in themselves, can generate earthquakes. Sackungen are post-glacial features found in heavily glaciated mountainous areas.

When ice filled the valleys, the interfluve ridge was essentially supported on both sides by glacial ice. With the melting of the glaciers, the over-steepened flanks caused the ridge to be out of gravitational equilibrium. Body forces within the ridge caused lateral spreading into the once ice-filled valleys. This can lead to the formation of ridge-top grabens and uphill-facing, normal faults on the flanks. Sackungen formation is thus commonly referred to as ‘ridge-top spreading’.

Diagram of sackung formation (Modified from Varnes, et al, 1989)

Along Bald Eagle Mountain, west of Leadville, geologists have measured slow spreading of sackungen by repeat measurements over several decades. Please see the article: Measurement of ridge spreading movements (Sackungen) at Bald Eagle Mountain. However, following a large earthquake on a nearby fault, a sackung in Alaska was observed to have experienced the creation of a fresh scarp from sudden movement that was triggered by the earthquake on a tectonic fault. This has led to an interest in trenching sackungen, to see whether they have any sudden displacements, in addition to the slow displacements. These sudden displacements could indicate large earthquakes on nearby tectonic faults.

The sackungen are good places for datable organic material because of their uphill-facing nature which can provide catchment for ponded sediments. The organic material could be a good source for radiocarbon dating of sudden events.

Map of Colorado Sackungen

Pictures of the sackungen for most of these locations are available here

Sackungen technical references – Colorado Geological Survey

Field Trip Guidebook on the Geology and Geologic Hazard of the Glenwood Springs Area

Sackungen at the Aspen Highlands Ski Area, Pitkin County, Colorado

Rocky Mountain Association of Geologists (RMAG) Guide to Rocky Mountain National Park